Breeding takes place in the spring and summer, and there is a single litter of about six kits which are reared exclusively by the female.

Due to its small size and fierce nature, the least weasel plays an important part in the mythology and legend of various cultures.

Based on skull characteristics, Reig (1997) proposed that the taxon should be split into four species, M. subpalmata, M. rixosa, M. vulgaris and M. eskimo.

[4] Within the genus Mustela, the least weasel is a relatively unspecialised form, as evidenced by its pedomorphic skull, which occurs even in large subspecies.

[6] The least weasel is the product of a process begun 5–7 million years ago, when northern forests were replaced by open grassland, thus prompting an explosive evolution of small, burrowing rodents.

The weasel's ancestors were larger than the current form, and underwent a reduction in size to exploit the new food source.

The least weasel thrived during the Ice Age, as its small size and long body allowed it to easily operate beneath snow, as well as hunt in burrows.

[7] The least weasel has a high geographic variation, a fact which has historically led to numerous disagreements among biologists studying its systematics.

hungarica (Vásárhelyi, 1942) minutus (Pomel, 1853) monticola (Cavazza, 1908) nikolskii (Semenov, 1899) occidentalis (Kratochvil, 1977) trettaui (Kleinschmidt, 1937) vasarhelyi (Kretzoi, 1942) The least weasel has a thin, greatly elongated and extremely flexible body with a small, yet elongated, blunt-muzzled head which is no thicker than the neck.

[21] The skull, especially that of the small rixosa group, has an infantile appearance when compared with that of other members of the genus Mustela (in particular, the stoat and kolonok).

The least weasel has muscular anal glands under the tail, which measure 7 by 5 mm (0.28 by 0.20 in), and contain sulphurous volatiles, including thietanes and dithiacyclopentanes.

The upper parts in the summer fur are dark, but vary geographically from dark-tawny or dark-chocolate to light pale tawny or sandy.

In the northern part of its range and at high altitudes, the least weasel changes colour in the winter, the coat becoming pure white and exhibiting a few black hairs in rare circumstances.

[1] The least weasel inhabits fields, open woodland, bushy and rocky areas, parks and gardens at elevations of up to about 3,000 m (9,800 ft).

In the Northern Hemisphere, the average litter size consists of 6 kits and these reach sexual maturity in 3 to 4 months.

Newborn kits are born pink, naked, blind and deaf, but gain a white coat of downy fur at the age of 4 days.

The nest chamber (which is used for sleeping, rearing kits and storing food) measures 10 cm (3.9 in) in diameter, and is lined with straw and the skins of the weasel's prey.

[33] The least weasel has four basic vocalisations; a guttural hiss emitted when alarmed, which is interspersed with short screaming barks and shrieks when provoked.

Exceptional cases are known of least weasels killing prey far larger than themselves, such as capercaillie, hazel hen and hares.

In years of scarcity, birds form a greater proportion of the diet and female least weasels may fail to breed.

Male least weasels take a higher proportion of rabbits than females, as well as an overall greater variety of prey.

This is linked to the fact that being larger, and having vaster territorial ranges than females, males have more opportunities to hunt a greater diversity of prey.

[37] The least weasel kills small prey, such as voles, with a bite to the occipital region of the skull[34] or the neck, dislocating the cervical vertebrae.

[38] Least weasel remains have been found in the excrement of red foxes, sables, steppe and forest polecat, stoats, eagle owls and buzzards.

Flea species known to infest weasels include Ctenophthalmus bisoctodentatus and Palaeopsylla m. minor, which they get from moles, P. s. soricis, which they get from shrews, Nosopsyllus fasciatus, which they get from rodents and Dasypsyllus gallinulae which they get from birds.

[38] Helminths known to infest weasels include the trematode Alaria, the nematodes Capillaria, Filaroides and Trichinella and the cestode Taenia.

[38] Least weasels are commonly infected with the nematode Skrjabingylus nasicola, adults of which are found in the nasal sinuses and can damage the skull.